The present invention relates to an optical disc player, and particularly, to an automatic control system for a tracking servo unbalance of optical disc player which determines automatically a value for making a tracking servo unbalance zero and storing this value digitally thereby eliminating tracking unbalance in an usual playback operation.
General in a compact disc, a signal is recorded on a track arranged helically in a direction from an inner side toward an outer side, of which distance between adjacent tracks; i.e., track pitch of a high density, is about 1.6 .mu.m.
Compact disc player (CDP) irradiates with a laser beam from a pick-up device to a disc by focusing thereto and converting the quantity of the light reflected therefrom into an electric signal, thereby reading the signal. It is required to control the position of the laser beam to be incident on the track of turning disc.
On the other hand, a motor for turning a compact disc causes a phenomenon that moves the track inwardly and outwardly at the time of turning and therefore, the pick-up follows this and a device controls to be positioned on a proper track which is called a tracking servo. A servo error signal for informing how much deviation from an optimum state occurred is required for all servos which the compact disc player applies various optical methods to produce, however a typical method of producing the tracking servo error is known as a three-beam system.
FIG. 1 is a diagram for illustrating a principle on a three-beam system of a conventional optical disc player. As shown in FIG. 1, laser beam from a light source is divided into a number of beams by utilizing a grating which is within the interior of pick-up, and thereafter a major beam M having an intensity of greatest light quantity is positioned on a track CH0 recorded with desired signal, and sub-beams E and F provided at both sides of major beam M are positioned so as only one part of it is superposed on the track CH0.
At this moment, the beams M, E and F reflected from the disc, as shown in FIG. 2, reach the photodetectors 1-3, and a current corresponding to the light intensity is produced for each beam. The currents produced from the said photodetectors 1 and 3 are respectively applied to a variable resistor VR1, a resistor R1 and inverse terminals of operational amplifiers OP1 and OP2, thereby being converted to voltages of magnitude proportional to those currents respectively. The variable resistor VR1 is used for adjusting the disproportion of current conversion of the photodetector 1 according to the arranging error of optical parts and the like, and the outputs of the operational amplifiers OP1 and OP2 are differentially amplified through an operational amplifier OP3 whereby being used as tracking serve error signal.
On the other hand, since the sub-beams E and F are partly irradiated onto the track CH0, the interference effect from the signals decreases the reflected light quantity. Assuming that the voltages converted by the operational amplifiers OP1 and OP2 are values of V.sub.EO and V.sub.FO, when the laser beams E, M and F are moved toward the direction A whereby being out of the track CH0 in this state, the beam E covers more of into the track CH0 and accordingly, the reflected light quantity is further decreased by the interference and dispersion of the signal whereby the produced current according to the photodetoctor 1 is decreased and converted into negative(-) sensitivity of voltage through the operational amplifier OP1, and therefore as shown in FIG. 3A, the magnitude is increased toward the arrow.
Simultaneously with this, the coverage of the beam F gets with respect to the track CH0 is decreased whereby the effect of the reflected light quantity according to the signal is reduced, and therefore the magnitude of voltage V.sub.FO converted into negative (-) component by the operational amplifier OP2 is decreased toward the arrow as shown in FIG. 3B. At this moment the tracking servo error of output by the operational amplifier OP3 is decreased toward the arrow 1, as shown in FIG. 3C.
When the beams E, M and F are further deviated toward A, the beam F covers the adjacent track CH1 and since the beam E gets no longer covers the track CH0, each reflected light quantity and conversion current and voltage accordingly is reversed with above-described case and therefore, the output tracking servo error is increased toward arrow 2, as shown in FIG. 3C. When the laser beam is further out of position and approaches the track CH1 closer than the track CH0, the coverage of the beam F with respect to the track CH1, is decreased and since the beam E covers the track CH1, the output tracking servo error is decreased toward arrow 3 as shown in FIG. 3C and finally arrives directly on the track CH1. That is, every time covering one track and covering the next track, the tracking servo error cycles one period of a sine wave, as shown in FIG. 3C.
The voltages V.sub.n and V.sub.p as shown in FIG. 3C affect the control power of the tracking servo, and the greater this magnitude, the stronger the control power is but if the voltages V.sub.EO and V.sub.FO are not the same by the optical parts or the photodetectors 1 and 3 and the like, the magnitudes of voltages V.sub.p and V.sub.n becomes unequal whereby an unbalance is produced in the control of the tracking servo.
Such unbalanced condition affects the stability of tracking servo, and particularly, when a factor of unstability occurs due to the external disturbance, it exerts a bad influence on the stability of the tracking servo and this is generally called an offset or an unbalance of tracking servo. In order to eliminate such unbalance, a variable resistor VR1 for determining the voltages V.sub.EO and V.sub.FO, the conversion sensitivity of the operational amplifiers OP1 and OP2, is adjusted and thereby controlled at the product adjusting process.
Consequently, in the tracking error adjustment process according to the three-beam system of conventional optical disc player, since the manual adjustment process is required in the production process, there have been disadvantages that the system is weak in viewpoint of productivity and cost, and that the required value of the variable resistor fluctuates according to the external vibration or temperature and the like.